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Article from Electronics & Music Maker, October 1981

Readers tested designs and improvements.


Readers contributions can be an original feature or constructional article describing electronic or electro-musical equipment which is ideally a complete building block that will aid the circuitmaker. All ideas and circuits presented for publication should have been proved before submission and should include a circuit diagram and explanatory text.

Low cost stereo LED meter


G. Durant, Brayton, N. Yorks

This design for a stereo LED meter differs from most current designs in that only one LED driver chip is used; this is multiplexed between the two stereo channels.

Each input is connected to a rectifier, followed by an RC network. This will act as a peak detector with a PPM type response. The 100R series resistor ensures that the tantalum (for low leakage) capacitor charges quickly, and hence catches short musical transients, and the 100k resistors allow this charge to leak away slowly thus giving the desired response.

The output from each capacitor is fed to a CMOS switch, and then to the LED driver. The CMOS switches are controlled by a D-type flip flop, one switch being connected to each output of the flip flop, thus ensuring that both switches cannot be 'on' together. The clock input of the D-type is fed from a 200Hz clock built around a traditional CMOS oscillator. It is obvious that each of the two inputs are connected to the LED driver in turn.

The LED driver may be any one of the three National drivers, LM3914/5/6 depending on the required response. Each output of the driver is connected to two LEDs; the common line of each row of LEDs being connected to the +ve rail via two more CMOS switches operated together with the input selector switches. Thus the right-hand column of LEDs lights when the right input is selected, and vice versa. Since this happens at 100Hz the eye perceives both lines to be on at the same time, the resultant saving of which is not merely a display driver but a considerable lessening in the number of wires required to connect the display and the driver.

There is also no reason why this approach cannot be expanded to have four or even eight audio input lines, where the cost and complexity saving will be considerable. If the design is to be used with high-power amplifiers some form of attenuation at the input will be required to avoid damaging the input amplifier with the high voltages. For lower power amplifiers or high signal levels the circuit may be used directly, the resistor Rx in the circuit being varied to obtain the correct calibration.

(Click image for higher resolution version)




True square wave oscillator



The standard CMOS oscillator shown suffers from the problem that the mark-space ratio is not necessarily balanced. This is simply due to differences in the input stages of the gate, resulting in slightly different switching points. This problem is usually of little consequence because most oscillators of this sort are used to drive edge-triggered circuits, the timing not being critical. However, in some multiplex designs, an equal mark-space ratio is required. This can be produced by adding another gate to the simple oscillator to provide feedback which drives the first of the oscillator gates hard, thus swamping any variations in the gate structure.





Guitar Tuner Mod


Michael Scott. Selby, N. Yorks

After seeing the Guitar Tuner project in E&MM (June 1981) I decided that it would be even more useful if it could be used for bass as well as lead guitars. The following modifications to the published design will achieve this quite cheaply with little modification to the PCB.

To accommodate the lower frequencies the cut-off of the input high-pass filter is lowered to 33Hz by changing C1 to 100nF. On the reference chain, R23-26 are removed, and replaced as follows:

R23, 24 replaced by R31-33 with the junction of R31,32 connected to the spare tag on S1 corresponding to G.

R25, 26 replaced by R39, 40 then R34-38 are soldered across the relevant spare tags on S1 (see diagram).

The PCB track connecting the pole of S1a and R27 is broken and connected to a new double pole switch, S2, together with a connection to the pole of S1b.

With S2 in 'lead' position the tuning is as before, but when 'Bass' is selected, it works as a bass guitar tuner using the voltages as listed in the table. The calibration is the same.

All the resistors added should be 1% metal oxide types.




Clean Gated Oscillator



It is necessary on many occasions in counters or similar equipment using pulse-trains, to ensure that the last pulse of a gated signal is not truncated. The circuit shown prevents such truncation in a very simple way-by adding a couple of gates around a standard oscillator. It works as follows: When the enable signal is low (active) the oscillator runs and the output is present. When the enable input is taken high, the output of the enable gate does not go low until the other input to the enable gate also goes high; since this input is connected to the oscillator output it is obvious that the oscillator will not stop until this occurs. When the oscillator stops, the outputs of both gates in the oscillator go high and this would result in a spurious pulse appearing on the output so an additional gate is used to combine the actual oscillator signals and the enable gate output such that the system output remains in its high state after the completion of the current low cycle once the enable is taken high.




Hexadrum Improvements


Lt. A. E. Wride, R.N., Portland, Dorset

The range of sounds produced by the Hexadrum (August 1981 issue of E&MM and various corrections) whilst being good on the high drums, lacks a certain something on the low drums. The case also produces mutual interference if the pads are struck just a little too hard and the pad layout can be improved for stage use.

To produce better resonance on drum 6 R64, R65 and C63, previously omitted, should be added to the circuit (R64 + R65 are 82k — C63 is 100nF). This involves soldering the three components together and then linking them to the appropriate place on the PCB, either by drilling holes or more simply by soldering them to existing components.

A better span on the drums can be produced by changing the values of C52 and C62 to 47nF and 33nF respectively as well as changing C42 to 22nF as detailed in Corrigenda September 1981 issue.

To produce a more even volume for each drum R18, R28, R38, R47, R57 and R67 can be adjusted to suit personal taste.

For stage use and to prevent mutual interference the pick-ups can be mounted on a piece of veneered chipboard. The box can be secured underneath using short wood screws having first recessed the chipboard for the pick-up wires.

The whole assembly can now be attached to a suitable stand or rested on an available speaker!


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Model Train Automatic Signal

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Using Microprocessors


Publisher: Electronics & Music Maker - Music Maker Publications (UK), Future Publishing.

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Electronics & Music Maker - Oct 1981

Feature

Previous article in this issue:

> Model Train Automatic Signal...

Next article in this issue:

> Using Microprocessors


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